Page:Popular Science Monthly Volume 45.djvu/261

Rh of earth movements we have another mere coincidence added to the long series already noticed. The depth of over twenty-five hundred feet undoubtedly seems enormous, but that depth exists just at the point where the two great valleys which have collected the converging streams above referred to unite together. Geologists will probably not think thirty thousand years an extravagant estimate for the duration of the Glacial period, in which case an erosion of only an inch in a year would be sufficient. Lago di Garda, the largest Italian lake, had a still larger catchment area in glacial times but not nearly so much concentrated; hence, perhaps, its comparatively moderate depth of about one thousand feet. We see, then, that on the theory of erosion, the size, depth, and position of the chief lakes are all intelligible, while on that of earth movements they have no meaning whatever, since the deep-seated agencies producing subsidence, upheaval, or curvature of the surface would be as likely to act in the small as in the large valleys, and to produce deep lakes in other places than those where, at a later epoch, the thickest glaciers accumulated.

—While Collecting facts for the present articles, it occurred to me that the rival theories of lake formation—erosion and submergence—were so different in their modes of action that they ought to produce some marked difference in the result. There must be some criteria by which to distinguish the two modes of origin. Under any system of earth movements a valley bottom will simply become submerged, and be hardly more altered than if it had been converted into a lake by building an artificial dam in a convenient situation. We should find, therefore, merely a submerged valley with all its usual peculiarities. If, however, the lake basin has been formed by glacial erosion, then some of the special valley features will have been destroyed, and we shall have a distinct set of characters which will be tolerably constant in all lakes so formed. Now I find that there are three such criteria by which we ought to be able to distinguish the two classes of lakes, and the application of these tests serves to show that most of the valley lakes of glaciated countries were not formed by submergence.

The first point is that valleys in mountainous countries often have the river channel forming a ravine for a few miles, afterward opening out into a flat valley, and then again closing, while at an elevation of a hundred or a few hundred feet, at the level of the top of the ravine, the valley walls slope back on each side, perhaps to be again flanked by precipices. Now, if such a valley were converted into a deep lake by any form of subsidence, these ravines would remain under water and form submerged river